Frequency division multiplexing

Frequency Division Multiplexing (FDM) refers to sharing a channel by dividing up the passband of the channel into multiple subbands. Each subband can carry a signal conveying data. The bandwidth (not the passband) allocated to a channel determines how much information the channel can carry; thus, subbands of equal bandwidth can each carry the same amount of information.

As long as the various input signals contain frequencies only in their designated subband, all the inputs can be transmitted simultaneously without interfering. This is the principle applied in broadcast radio. Each station is assigned its own transmitting frequency.

FDM is associated with analog transmission facilities. It transports a number of narrow passband signals over a single wideband facility. Within each channel, analog or digital information can be carried. If the information is analog, a direct modulation technique can be used. If the information is digital, a carrier modulation technique is employed. The diagram on the facing visual shows a facility with a 20 kHz bandwidth being divided into five 4 kHz passbands. If this was a facility associated with the PSTN, each passband could carry a single voice conversation whose bandwidth requirement does not exceed 3 kHz.

In most FDM systems, the channels are not as closely placed as depicted on the visual. A small space, called a guardband, is placed between the individual channels to minimize the probability of one channel “bleeding” into the other.

FDM is not just associated with copper facilities. Many wireless technologies also use FDM to separate users. In a fiber-optic transmission system, a technique called wavelength division multiplexing (WDM) is used. The frequencies are denoted by the greek letter lamda and the passbands are enormous; they can transport up to 160 Gbps per passband.

The words wavelength and frequency are very closely related. Consider the waves generated in the wave pool of your local amusement park. The waves leave the wave generator and move at a reasonably constant rate to the “beach” of the pool. Assume the waves are moving one foot per second. If the wave generator is set to generate a new wave every 10 seconds, we could measure the distance from the top of one wave to the top of the next and find a wavelength of 10 feet. Alternatively, we could stand still and count the waves as they pass us. We would note six waves pass every minute, so the frequency is 6 waves per minute. Now reset the wave generator to generate a wave every 5 seconds. The waves still move through the water at a fixed speed, which has not changed from the previous example. They are simply being generated more rapidly. Measuring from the top of one wave to the next we find the wavelength is now 5 feet. Standing still and observing the passage of the waves we not that the frequency is now 12 waves per minute. For a fixed propagation delay (i.e., the speed the waves move through the water), halving the wavelength doubles the frequency. So the frequency of a signal and its wavelength are simply two ways of describing the same wave.